An evaluation of gillnet CPUE as an index of perch density in small forest lakes

Gillnets are commonly used for monitoring fish communities and assessing the ecological quality inEuropean lakes. However, the relationship between gillnet CPUE and fish abundance is not linear anddepends on several factors affecting the catchability of gillnets. We investigated the effects of high watercolour, naturally anoxic hypolimnion, water temperature and sex ratio on the catchability of perch Percafluviatilis in gillnets. Data were collected during 10 years in nine forest lakes in southern Finland. Suchlakes are very common in Finland. The density and size structure of perch populations were estimatedby mark and recapture method during spring and standard gillnet fishing (CEN, 2005. Water qualitySampling of Fish with Multimesh Gillnets. European Committee for Standardization, Brussels (EN 14757))in late summer. Perch CPUE was positively but non-linearly dependent on the perch density. Catchabilityincreased with water colour, water temperature, and the volume of the anoxic hypolimnion. The sex ratiowas also important because late summer gillnetting overestimates the proportion of female perch. If thefactors that affect the catchability of perch in gillnets are taken into account, gillnet CPUE using standardmethods can be used as rough index of fish density in small forest lakes.Peer reviewe

The capability of harvestable slot-length limit regulation in conserving large and old northern pike (Esox lucius)

In this experimental fisheries regulation study, we compared the effects of size-selective fishing according to minimum length limit (MLL, 40 cm) or harvestable slot-length limit (HSL, 40–64.9 cm) on population density, biomass, size and age structures, and sex ratio of the northern pike (Esox lucius). Data were collected during the years 2006–2013 in four 2.1–13.8 ha pristine forest lakes in southern Finland. In lakes where MLL fishing was carried out, pike density, biomass and size structure showed greater negative responses than in lakes where HSL fishing was carried out: mean length and age decreased, and large pike (≥ 65 cm) disappeared in just four years. HSL fishing affected the pike to a lesser extent, and large pike remained in the lakes, which is considered an essential feature of sustainable fishing. However, the results suggest that the number of large pike may decrease even under the HSL fishing strategy if the growth rate is slow and fishing is intensive, due to the high possibility of fish being caught before reaching the length beyond the upper HSL limit.Peer reviewe

Effects of non-selective and size-selective fishing on perch populations in a small lake

Retaining large individuals is considered intrinsic to sustainable fishing. In this nine-year study, we explored the effects of simulated recreational fishing on life-history traits of two perch (Perca fluviatilis) populations in a lake divided for experimental purposes into two sections. In each section, one of the two following fishing methods was used: non-selective and negatively size-selective i.e. large individuals released. Non-selective fishing rapidly decreased the average size and age of the spawning perch stock thus reducing the average size of spawned eggs. Both fishing procedures increased the share of females in the spawning population due to decreased age at maturity. The average age at maturity decreased more in females than in males. The reductions in the density and biomass of the populations and increase in the growth rate of perch were temporary but the effects on size and age structure persisted throughout the study period. The retention of large individuals can delay the adverse effects of fishing on populations, and enable reproduction of large females, thereby sustaining high genetic variability and better quality of offspring.Peer reviewe

Zooplankton rhapsody: Unexpected response in community following increased fish predation

The impact of increased fish density on the crustacean zooplankton was studied in a small, forest lake. Interestingly and contrary to our hypothesis, increased fish density resulted in an increase in biomass of large-sized cladocerans, but a decrease in biomass of smaller and dominant cladoceran species. Thus, the effect of increased planktivory of fish was likely tempered by compensatory dynamics of zooplankton, wherein abundance of some species increased to compensate for population decreases by other species. This change in the relative species composition of zooplankton was seen as the increased diversity in crustacean zooplankton community. We conclude that instead of simple “top-down” or “bottom-up” regulation of zooplankton, the diverse array of connections among species is more likely to cause community dynamic fluctuations in zooplankton. Moreover, behavioural changes e.g. habitat change of fish may also have a pronounced effect on the response of zooplankton community on fishing.Peer reviewe

Preface to the Special Issue “Ecologically sustainable fishing” : Project KESKALA background, objectives and conclusions

Non peer reviewe

Roach (Rutilus rutilus) populations respond to varying environment by altering size structure and growth rate.

To better understand the environmental factors and biological interactions affecting roach abundance and population structure in oligo-mesotrophic lakes, we explored roach population parameters in 24 small forest lakes in southern Finland. In those lakes, roach abundance was only little affected by the abiotic and biotic variables such as lake characteristics, water transparency, nutrient concentrations, pH or related variables, predation and competition. This was probably due to the generalist characteristics of the roach as it is adaptive to a wide variety of environments. Instead, the population structure and age-specific growth of the roach were strongly affected by various environmental variables. The growth rate was negatively related to water colour, calcium and nutrient concentration and roach abundance, and positively related to perch abundance and lake size. The growth patterns and relatively stable abundance showed flexibility in adaptation to different environmental conditions. Our results indicate that the ongoing environmental changes, recovery from acidification and brownification may notably affect the structure and production of roach populations, and may alter the role of the roach in lake ecosystems.Peer reviewe